This invention relates to communications networks and in particular to the construction and use of filters for coupling telephone sets to a subscriber loop. The invention further relates to a method of interference suppression in a subscriber""s telephone installation.
Many telephone subscribers have a number of telephone sets coupled in parallel to a single subscriber loop so that the subscriber can make and receive calls at a number of locations within his premises. In normal use, only one telephone set will be in use at any one time, the other sets remaining on hook.
Many telephone subscribers now have data facilities, for example ADSL or VDSL services coupled to the subscriber loop via a modern at the subscriber station. These moderns use frequencies above the normal voice or POTS frequencies so that normal telephony can co-exist on the subscriber loop with the data service. It will, of course, be appreciated that telephone designs take no account of input impedance above the normal POTS frequency of about 4 kHz. This has been found to present problems when the phone is used on a line carrying data traffic. When the telephone is off-hook, the data signal can get into the telephone and, as a result of non-linearity, be converted down into the voice band so as to appear as noise on the line. Further, the impedance of the line changes at these higher frequencies altering the telephone hybrid balance and hence the near end cross-talk (NEXT) response. In addition, in the process of going from on-hook to off-hook, the current drawn from the line can cause a high frequency transient that causes errors in the data transmission.
To address these problems, it has been proposed to provide a telephone set with a filter, typically an LC filter, to remove or block unwanted voice band frequencies. It will be appreciated, however, that the characteristics of such a filter are dependent upon the on-hook or off-hook condition of the telephone set to which that filter is connected. When the telephone set is off-hook, the low impedance of the telephone set correctly terminates the filter which can then perform its intended function of blocking data signals from the telephone voice circuits. However, when the telephone set is in its on-hook condition, the filter is presented with a very high terminating impedance. This introduces the undesirable characteristic of very low input impedances at frequencies determined by the values of the inductors and capacitors used to construct the filter. This has now become a significant problem because of the increasingly common practice of coupling a number of telephone sets to a subscriber loop in parallel in a subscriber installation. The filters coupled between each telephone and the subscriber loop will also be effectively arranged in parallel and, apart from the particular telephone set that is currently in use in its off-hook condition, the remaining filters will be effectively unterminated. These unterminated filters will thus present very low impedances at voice frequencies thus impairing the effective operation of the off hook telephone set currently in use.
An object of the invention is to minimise or to overcome the above disadvantage.
A further object of the invention is to provide an improved filter arrangement whereby to facilitate the parallel operation of a number of telephone sets.
A further object of the invention is to provide an improved method of interference suppression in a telephone subscriber""s installation.
According to a first aspect of the invention, there is provided a filter arrangement for coupling a telephone set to a subscriber loop, the arrangement comprising a filter circuit for rejecting selected frequencies from the telephone set, said filter being so constructed as to perform its filtering function only when the telephone set to which it is connected is in its off-hook low impedance condition.
According to a further aspect of the invention, there is provided a filter arrangement for coupling a telephone set to a subscriber loop, the arrangement comprising a filter circuit incorporating series connected inductive and parallel connected capacitive elements for rejecting selected frequencies from the telephone set, and switch means responsive to current drawn from the line for selectively enabling and disabling the filter arrangement when the telephone set is respectively in its off-hook condition or in its on-hook condition.
According to another aspect of the invention, there is provided a subscriber""s telephone installation, comprising a plurality of telephone sets coupled in parallel to a subscriber loop so as to provide a voice facility, and a data modern coupled to the subscriber loop so as to provide data facilities at a frequency above that employed for said voice facility, wherein some or all said telephone sets are provided each with a respective filter arrangement for rejecting selected frequencies, each said filter arrangement being so constructed as to perform its filtering function only when the telephone set to which it is connected is in its off-hook low impedance condition.
According to another aspect of the invention, there is provided a subscriber""s telephone installation, comprising a plurality of telephone sets coupled in parallel to a subscriber loop so as to provide a voice facility, and a data modern coupled to the subscriber loop so as to provide data facilities at a frequency above that employed for said voice facility, wherein some or all said telephone sets are provided eachwith a respective filter arrangement incorporating series connected inductive and parallel connected capacitive elements for rejecting selected frequencies from the telephone set, and switch means responsive to current drawn from the line for selectively enabling and disabling the filter arrangement when the telephone set is respectively in its off-hook condition or in its on-hook condition.
According to a further aspect of the invention, there is provided a method of interference suppression in a subscriber""s telephone installation, comprising a plurality of telephone sets coupled in parallel to a subscriber loop so as to provide a voice facility, and a data modern coupled to the subscriber loop so as to provide data facilities at a frequency above that employed for said voice facility, wherein some or all said telephone sets are provided each with a respective filter arrangement for rejecting selected frequencies; the method comprising enabling that filter associated with a said telephone only when that telephone is in use its off-hook low impedance condition.